Ultra-small accelerometers have drawn much research interest due to their low cost, high performance, and the miniaturization trend of a variety of electronic devices used in automobiles, military systems, robot systems, and safety diagnostic systems. Among the ultra-small accelerometers, capacitive accelerometers measure the acceleration generated due to an external force by measuring a change in capacitance due to the acceleration, between a reference electrode and a sensing electrode.
Capacitance C is defined by Equation 1, and increases as a distance d between the electrodes decreases.
                    C        =                  ɛ          ⁢                                    A              d                        .                                              (        1        )            
Also, since a change in the capacitance C when the distance d between the electrodes is small is higher than that when the distance d between the electrodes is large, the smaller the distance d is, the more sensitive the accelerometers are. Accordingly, when ultra-small capacitive accelerometers are manufactured, it is very important to precisely reduce a distance between electrodes.
FIG. 1 is a plan view of a conventional ultra-small capacitive accelerometer manufactured by bulk micromachining.
Referring to FIG. 1, a substrate 64 is selectively etched to form sensing mass bodies 130 and 140 including sensing electrodes 110 and 120 and support springs 132 and 142, support parts 136, 138, 146, and 148 fixing the support springs 132 and 142 to the substrate 64 and acting as pads for applying signals, reference electrodes 80 and 90 spaced apart by a predetermined distance from the sensing electrodes 110 and 120, and pads 88 and 98 applying signals to the reference electrodes 80 and 90.
When acceleration is generated by an external force, the sensing mass bodies 130 and 140 are moved by an inertial force, and thus the distance between the sensing electrodes 110 and 120 and the reference electrodes 80 and 90 is changed. As a result, the capacitance between the sensing electrodes 110 and 120 and the reference electrodes 80 and 90 is changed and the acceleration can be measured according to the changed capacitance.
However, the conventional ultra-small capacitive accelerometer has a problem in that as an aspect ratio of the electrodes increases, a minimum distance between the electrodes is limited by a manufacturing process. Also, the distance between the sensing electrodes 110 and 120 and the reference electrodes 80 and 90 may be different for each manufactured accelerometer due to a process error, such as overetching, and thus each accelerometer must be individually corrected.
FIG. 2A is a perspective view of a conventional highly sensitive capacitive accelerometer manufactured by both bulk micromachining and surface micromachining. FIG. 2B is a cross-sectional view taken along a line perpendicular to the length direction of sensing electrodes 41 of the conventional highly sensitive capacitive accelerometer of FIG. 2A.
Referring to FIGS. 2A and 2B, sensing mass bodies 42 are formed by bulk micromachining, and support springs 51 and sensing electrodes 41 are formed by surface micromachining. A distance between the sensing electrodes 41 and the sensing mass bodies 42 is determined by the thickness of a deposited sacrificial layer, not shown, which surrounds the sensing electrodes 41 and is removed in FIG. 2B. The distance between the sensing electrodes 41 and the sensing mass bodies 42 can be narrowed. Also, since an entire substrate 40 is etched, sensitivity can be increased by sufficiently increasing the mass of the sensing mass bodies 42.
However, a method of manufacturing the conventional highly sensitive capacitive accelerometer is very complex and expensive, and has a high risk of causing a difference in sensitivity between different accelerometers due to a process error because it is difficult to precisely and uniformly control the thickness of the sacrificial layer.
Accordingly, there is a high demand for an accelerometer that can be simply manufactured at low cost and has a low risk of causing a difference in sensitivity between devices.